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iScience
iScience
iScience
2589-0042
Elsevier

S2589-0042(24)01801-7
10.1016/j.isci.2024.110576
110576
Article
Experimental study on the degradation of sandstone-petroglyph carriers in Helankou: Effects of freeze-thaw and wet-dry cycles
Kong Xiangchao 12
He Faguo hefg@lzu.edu.cn
123∗
Lv Ran 12
Zhang Lizhi 12
Yang Xin 12
Lu Tengfei 12
1 College of Civil Engineering and Mechanics, Lanzhou University, Lanzhou 730000, China
2 Key Laboratory of Mechanics on Disaster and Environment in Western China with the Ministry of Education, Lanzhou University, Lanzhou 730000, China
∗ Corresponding author hefg@lzu.edu.cn
3 Lead contact

25 7 2024
16 8 2024
25 7 2024
27 8 11057621 4 2024
23 6 2024
22 7 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Summary

The Helankou petroglyphs offer precious data for understanding the lifeways of ancient pastoralists. However, in Helankou, the combination of unique climate conditions and complex hydrochemical environments has accelerated the weathering of the sandstone-petroglyph carriers, leading to numerous petroglyphs at risk of vanishing. In response, this study employed freeze-thaw and wet-dry cycle experiments to simulate the actual environmental conditions for sandstone. Subsequently, the sandstone that underwent the experiments was subjected to further testing and analysis. The goal was to comprehensively understand the performance change patterns and micro-mechanisms in sandstone under environmental stress, thus providing scientific theoretical support for Helankou petroglyph conservation. The results indicate that under the effects of freeze-thaw and wet-dry, the degradation processes can be attributed to a diverse combination of mechanisms such as swelling-shrinkage, hydrolysis, frost heave, dissolution, and salt crystallization, which alter the physical parameters, particle size distribution, mineral composition, and structural integrity of the sandstone.

Graphical abstract

Highlights

• Freeze-thaw, wet-dry, and chemical coupling critically degrade petroglyph sandstone

• Simulated Helankou petroglyphs’ environment to explore degradation mechanisms

• Offers scientific theoretical support for Helankou petroglyphs conservation

Mineralogy; Weathering; Archeology

Subject areas

Mineralogy
Weathering
Archeology
Published: July 25, 2024
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pmcIntroduction

Helankou is one of the world’s most densely populated areas for petroglyphs, known globally for its diverse artistic expressions and profound cultural significance. However, due to the open-air preservation of these petroglyphs, drastic environmental changes have severely weathered the sandstone surfaces that carry them. This weathering has caused a variety of damage to the petroglyphs, including shallowing carvings, sheet-like warping, and detachment, which poses a significant risk of disappearance. Furthermore, the unique climatic features and complex hydrochemical conditions in Helankou have made freeze-thaw cycles (FTCs),1 moisture variations,2 and chemical erosion3 significant factors in the weathering of the sandstone-petroglyph carriers. Consequently, understanding the impact of the existing environment on the degradation mechanisms of the carrier sandstone and developing scientific preservation measures has become critically urgent.

FTCs, acting as a dynamic coupling effect of alternating high and low temperatures, are a key factor in inducing creep in rocks and causing damage.4 Researchers systematically assessed the impact of FTCs on the mechanical properties of various rocks (such as andesite, sandstone, limestone, dolomite, and basalt), utilizing a range of tests including uniaxial compressive strength tests,5,6,7,8,9,10,11,12 point load strength index tests,3,13 shear tests,14 and triaxial tests.15,16,17 Findings indicate that key mechanical parameters of rocks (such as compressive strength, tensile strength, cohesion, internal friction angle, and elastic modulus) exhibit an exponential decline as the cycle count increases. Furthermore, Yang et al.18 studied the failure modes and processes of marble, granite, and sandstone after FTCs, finding that marble has the strongest resistance to freeze-thaw weathering, while sandstone has the weakest. To further comprehend the impact of FTCs on rock structure, researchers utilized sophisticated non-destructive examination methods such as nuclear magnetic resonance (NMR),19,20 X-ray diffraction (XRD),21,22 X-ray computed tomography (CT),23 scanning electron microscopy (SEM),1,24 and velocity measurements11,17,25 to analyze changes in the microstructure of rocks. These analyses revealed that as the number of FTCs increases, there is a decrease in the P-wave velocity in rocks, and an augmentation in both porosity and permeability, implying that FTCs trigger modifications in the dynamic attributes of rocks.26,27 Wet-dry cycles , which represent a periodic hydro-rock interaction, inflict damage on rocks via a “repetitive softening” effect, which often leads to more severe damage than continuous soaking.28,29,30 Currently, scholars commonly employ a “soaking in water followed by oven drying” wet-dry cycling process to expedite the weathering of rock materials, focusing on the patterns of change in the physical and macro-mechanical properties of rocks.31,32,33,34 Simultaneously, the study delves into the impact of wet-dry cycles on metamorphic phenomena against the backdrop of changes in the physical properties of rocks.35,36,37,38 Findings demonstrate that wet-dry cycles increase the effective porosity volume and improve pore connectivity, thereby exacerbating the accumulation of rock damage. Moreover, Song et al.39 through research using NMR technology, found that although damage increases with the number of wet-dry cycles , the rate of damage growth eventually stabilizes. The discoveries previously offer systematic understanding of how freeze-thaw and wet-dry cycles affect rock structures and performance. Meanwhile, water, as a major factor affecting wet-dry cycles and FTCs, cannot be overlooked. Water typically exists in natural environments as complex chemical solutions containing multiple ions and varying pH levels, which cause varying degrees of corrosion to rocks.24 Researchers have investigated the effects of mechanical property damage to rocks after freeze-thaw3,40,41,42 and wet-dry cycles43,44 in various chemical solutions, finding that compared to single environmental conditions, the coupling action of environmental factors significantly accelerates the damage process of materials. The research also extensively examines how different chemical components specifically affect the mechanisms of damage. However, in the Helankou area, certain sandstones are affected by the freeze-thaw action of solutions with the same solute under different pH conditions and the wet-dry cycles caused by precipitation with varying pH. Current research on this phenomenon remains relatively scarce. Moreover, the existing research on the weathering of Helankou sandstones under actual environmental conditions is also insufficient.

Therefore, based on the actual environmental conditions of the sandstone carrying the petroglyphs in Helankou, this paper designed four groups of accelerated weathering tests for the sandstone samples, namely: the dry freeze-thaw, the water-saturated freeze-thaw, the salt-resistant freeze-thaw (utilizing solutions of the same solute at different pH levels), and the wet-dry cycling. Tests on the mass, P-wave velocity, and hardness of the samples were conducted following various cycles, complemented by integrated analyses of particle size distribution, scanning electron microscopy (SEM), and XRD. The objective is to comprehensively analyze the patterns of performance changes and the mechanisms of chemical damage in sandstones under environmental stress, from both macroscopic and microscopic perspectives. This research aims to provide theoretical support and practical references for developing scientific conservation strategies for the Helankou petroglyphs.

Environmental context of petroglyphs and experimental design

Environmental context of petroglyphs

Helankou (Figure 1) is located in Yinchuan City, Ningxia Hui Autonomous Region, China, characterized by a temperate continental climate. The annual average rainfall in this area is about 430mm, which is mainly concentrated during July and August and includes occurrences of acid rain corrosion. The annual average temperature is −0.8°C, with historical extremes of 39°C and −31°C, resulting in an extreme temperature range of 70°C.45 Furthermore, the area records over 3040 h of annual sunshine and an average evaporation rate of about 1600 mm, placing it among the regions with the highest levels of sunshine duration and evaporation in China.46 Additionally, Helankou features perennial streams originating from spring water, with a complex chemical composition predominantly including ions such as H+, Na+, OH−, and SO42−. Consequently, the intense seasonal temperature fluctuations, periodic rainfall (including acid rain), high evaporation rates, and other changes in the wet-dry freeze-thaw environment, along with the complex hydrochemical environment, have led to the weathering of the sandstones bearing petroglyphs. The primary manifestations of this weathering are the shallowing of petroglyph grooves and the sheet-like warping and spalling (Figure 2).Figure 1 Helankou location diagram

Figure 2 Main types of damage to the Helankou petroglyphs

Sample preparation

The sandstone used in this study was sourced from the native rock mass surrounding the Helankou petroglyphs. Following standard requirements,47 the rock blocks were cut and polished into standard cylindrical samples with a height of 50 mm and a diameter of 50 mm (adjacent surface angles of 90° ± 0.25°), discarding any samples with visible defects. Wave speed measurements were conducted along the axial direction of the samples, followed by the selection of two sets of samples with similar wave speeds (wave speed variation within each set ±100 m/s) to be utilized in freeze-thaw and wet-dry cycling tests. The physical properties of these samples were then systematically evaluated according to standards,47 as shown in Table 1.Table 1 Basic physical parameters of sandstone

Rock Category	Natural Density (g/cm³)	Dry Density (g/cm³)	Natural Water Content (%)	Natural Absorption Rate (%)	Water Absorption Rate (%)	
Sandstone	2.72	2.70	0.89	0.11	0.15	

Experimental design

Based on the integrated characteristics of the environment where the sandstone with petroglyphs in Helankou is located, it is planned to subject the samples to freeze-thaw and wet-dry cycling tests. Additionally, a control group is established using the untreated sample. The freeze-thaw cycling tests are divided into dry, water-saturated, and salt-resistant freeze-thaw groups. The dry freeze-thaw group simulates the impact of external temperature changes on sandstone degradation. The temperature is set according to standard requirements47 combined with historical extreme temperature records of Helankou, ranging from −31°C to 40°C. The water-saturated and salt-resistant freeze-thaw groups examine the impacts of water, salt, and temperature variations on sandstone. Soluble salt Na2SO4 and distilled water are chosen as the soaking solutions based on the water environment characteristics of Helankou, with the Na2SO4 solution at a concentration of 0.5 mol/L, and samples are vacuum-pumped before soaking. The wet-dry cycle group studies the effects of changes in internal water content and the combined effects of external environmental pH and temperature on sandstone. It is designed to simulate the rapid evaporation following a downpour. Based on historical rainfall pH monitoring, the wet-dry cycling tests are conducted using an acidic solution with a pH of 5 and distilled water with a pH of 7 as the soaking solutions. The drying evaporation temperature is set at 40°C based on historical extreme temperatures.

Depending on the experimental conditions, the four groups of experiments were further divided into seven subgroups (1-1, 2-1∼2-4, 3-1∼3-2), each consisting of three parallel samples to ensure the reliability of the results. The total number of cycles was set at 72, with each cycle lasting 24 h. After varying numbers of cycles, the samples were tested for mass, P-wave velocity, and surface hardness. Upon the completion of the 72 cycles, comprehensive analyses including particle size distribution, scanning electron microscopy (SEM), and XRD were conducted on the samples. The results of the accelerated weathering process are detailed in Table 2.Table 2 Experimental degradation process setup

Experimental Grouping	Subgroup Number	ID	Variable Factors	Sample Status	Degradation Process	
Drying-Freeze-Thaw Cycle Group	1	1-1	Temperature	Drying	Dried 12 h at 40°C oven.
Frozen 12 h at −31°C.	
Water-Saturated Freeze-Thaw Cycle Group	2	2-1	Temperature	Water Saturation	Immersed 12 h in 40°C, pH = 7 distilled water.
Frozen 12 h at −31°C.	
Salt-Resistant Freeze-Thaw Cycle Group	3	2-2	Temperature and pH	Water Saturation	Immersed 12 h in 40°C, pH = 3 Na2SO4 solution.
Frozen 12 h at −31°C.	
4	2-3	Temperature and pH	Water Saturation	Immersed 12 h in 40°C, pH = 7 Na2SO4 solution.
Frozen 12 h at −31°C.	
5	2-4	Temperature and pH	Water Saturation	Immersed 12 h in 40°C, pH = 12 Na2SO4 solution.
Frozen 12 h at −31°C.	
Wet-Dry Cycle Group	6	3-1	Temperature	Water Saturation to Drying	Immersed 12 h in pH = 7 distilled water.
Dried 12 h at 40°C oven.	
7	3-2	Temperature	Water Saturation to Drying	Immersed 12 h in pH = 5 acidic solution.
Dried 12 h at 40°C oven.	

Experimental results

Changes in mass

Mass change is the most direct manifestation of damage or destruction of samples during the testing process. To assess this, the mass of samples subjected to drying, water-saturated, salt-resistant FTCs, and wet-dry cycles was measured (Figure 3A), and the mass loss rate every three cycles was calculated (Figure 3B). The findings indicate that, following the cycles under varied conditions, the mass of all seven sample subgroups decreased to differing extents. Specifically, Subgroups 2-1, 3-1, and 1-1 had lower rates of mass loss, while 2-2, 3-2, and 2-4 experienced higher rates of mass loss. Notably, mass changes for samples in the drying and water-saturated freeze-thaw groups were smaller compared to those in the salt-resistant freeze-thaw group. In the salt-resistant freeze-thaw group, the highest rate of mass loss throughout the cycle was observed in samples soaked in acidic solutions, followed by those in alkaline solutions, and finally, in neutral solutions. In the wet-dry group, the mass loss rate of samples soaked in acidic solutions was higher than those soaked in distilled water solutions.Figure 3 Mass-cycle relationship

(A) The relationship between changes in sample mass and cycle numbers under various freeze-thaw and wet-dry conditions, with the inset in the lower left displaying the mass of the sample after drying in its initial state. Data are represented as mean ± SD.

(B) The variation in mass loss rate of samples with the number of cycles under the same experimental conditions. Data are represented as mean ± SD.

Changes in P-wave velocity

During the degradation process, samples often develop structural defects such as micro-cracks or microscopic deformations, which are indicative of internal structural changes. This is assessed by testing the P-wave velocity to assess the rock’s internal structure. The P-wave velocities of samples subjected to drying, water-saturated, salt-resistant freeze-thaw cycling, and wet-dry cycling were measured to determine the changes in P-wave velocity (Figure 4A) and to calculate the rate of velocity loss for every six cycles (Figure 4B). The results showed that, after undergoing various cycles, the P-wave velocities of the seven sample subgroups changed systematically, i.e., the P-wave velocities demonstrated fluctuations and attenuation as the number of cycles increased. Among them, Subgroups 2-1, 1-1, and 3-1 had a lower rate of velocity loss, while subgroups 2-2, 3-2, and 2-4 had a higher rate of velocity loss. Throughout the entire cycle period, the ranking based on the extent of change in P-wave velocity for samples under different cycling conditions was essentially consistent with the rate of mass loss.23Figure 4 P-wave velocity-cycle relationship

(A) The relationship between changes in P-wave velocity of the sample and cycle numbers under various freeze-thaw and wet-dry conditions, with the inset in the lower left displaying the P-wave velocity of the sample in its initial state.

(B) The variation in P-wave velocity loss rate of samples with the number of cycles under the same experimental conditions.

Changes in surface hardness

As the weathering of the rock surface progresses, the development and expansion of numerous microcracks, their fusion and interconnection, as well as the dissolution and loss of intergranular cementing materials, all lead to a loosening of the rock surface structure. Therefore, the degree of surface weathering can be characterized by the changes in the sample’s surface hardness. This test employs a portable Leeb hardness tester, using a single-point impact method, to measure the Leeb hardness of sample surfaces that have undergone drying, water-saturated, salt-resistant FTCs, and wet-dry cycles, with the results being shown in Figure 5A. The trend of hardness loss for the samples every three cycles is illustrated in Figure 5B.Figure 5 Surface hardness-cycle relationship

(A) The relationship between changes in surface hardness of the sample and the number of cycles under various freeze-thaw and wet-dry conditions, with the inset in the lower left showing the surface hardness of the sample in its initial state. Data are represented as mean ± SD.

(B) The variation in hardness loss rate of the sample with the number of cycles under the same experimental conditions. Data are represented as mean ± SD.

The results show that after cycles under various conditions, the surface hardness of samples from seven subgroups all displayed a fluctuating downward trend as the number of cycles increased. Following 72 cycles, the ranking of the extent of decrease in hardness of each subgroup of samples is essentially consistent with the ranking of mass and wave velocity loss rates. Simultaneously, the aforementioned three test results reveal that although the decline in sample mass, surface hardness, and P-wave velocity followed a broadly similar pattern, the magnitudes of decline varied significantly. The rate of decrease in surface hardness was the fastest, followed by P-wave velocity, indicating that the degradation of the samples proceeded from the outside inwards, progressing gradually from the surface to the interior.48,49 Meanwhile, the mass, influenced by the water content, decreased at the slowest rate.

Figure 6 depicts the relationship between P-wave velocity and surface hardness of sandstone under different cyclic conditions. The surface Leeb hardness of samples under varied cyclic conditions demonstrates a positive trend with the increment of P-wave velocity (VP). The fitting coefficient R2 is consistently above 0.93 for all, indicating a good quantitative correspondence between the surface hardness and P-wave velocity of sandstone samples under different cyclic conditions.Figure 6 P-wave velocity-surface hardness relationship

(A) Under dry and water-saturated freeze-thaw conditions.

(B) Under freeze-thaw conditions in a Na₂SO₄ salt solution with pH 3 and pH 7.

(C) Under freeze-thaw conditions in a Na₂SO₄ salt solution with pH 12.

(D) Under wet-dry conditions in acidic solution and distilled water.

Changes in apparent features

Figure 7 presents the seven subgroups of samples before and after the experiment. From the left side of Figure 7, it can be seen that the samples before the experiment have distinct edges, a compact structure, and a smooth and even upper surface. From the right side of Figure 7, it is known that among the seven subgroups of samples after 72 cycles of degradation under different conditions, the surface of subgroup 1-1 samples showed powdering and flaking, with significant particle shedding at the edges and continuous tiny notches; micro-cracks developed on the surface of 1-1-2. For subgroup 2-1 samples, block spalling can be seen in parts of the edges, and micro-cracks are visible on the surface. The micro-cracks, particle shedding, and block spalling of subgroup 2-2, 2-3, and 2-4 samples were more developed; the surface color became duller and darker, with dense tiny pits visible locally, and white spots and dark patches appeared on the upper surface. Subgroup 3-1 samples had slight particle shedding near the edges. For Subgroup 3-2 samples, particle and block flaking were more obvious on the upper surface; with tiny pits visible on the local surface and numerous micro-cracks developed. The phenomena described previously suggest that the action of salts and changes in environmental pH are the primary causes of sandstone degradation.24,50Figure 7 Apparent structural characteristics of samples before and after experiments under different conditions

1-Particle shedding; 2-Crack; 3-Block spalling; 4-Deep spot.

(A) Under dry freeze-thaw conditions.

(B) Under water-saturated freeze-thaw conditions.

(C–E) Under freeze-thaw conditions in a Na₂SO₄ salt solution with (C) pH 3, (D) pH 7, and (E) pH 12.

(F) Under wet-dry conditions in distilled water.

(G) Under wet-dry conditions in an acidic solution.

Particle size analysis

In order to precisely compare the variations in particle composition and content of sandstone under varying environmental conditions, samples were taken from the surface of each set of specimens and tested, with the particle size distribution curve shown in Figure 8.Figure 8 Changes in particle size distribution of the sample

(A) Under dry and water-saturated freeze-thaw conditions.

(B) Under freeze-thaw conditions in a Na₂SO₄ salt solution.

(C) Under wet-dry conditions in acidic solution and distilled water.

After 72 cycles of freeze-thaw, the particle size distribution of the samples altered relative to the untreated samples, with a general decrease in the volume fractions of the cementing material (particle size <0.03 mm), medium sand (particle size 0.25–0.5 mm), and coarse sand (particle size 0.5–1.0 mm), and an increase in the volume fractions of fine sand (particle size 0.10–0.25 mm) and very fine sand (particle size 0.03–0.10 mm). The loss of large particles (medium and coarse sand) and the increase in small particle sizes (fine and very fine sand) during the FTCs led to a decreasing trend in the median particle size d50. It can be observed that the particle size distribution of the sandstone samples after the FTCs becomes relatively uniform, which is consistent with the findings of ZHANG et al.51 regarding the influence of FTCs on the particle size composition of morainic clay. After 72 wet-dry cycles, the pattern of particle size distribution changes was analogous to that observed after the freeze-thaw experiment, indicating that the particle size distribution also evolved toward relative uniformity as the wet-dry test progressed.52,53

Microstructural analysis

To ascertain the changes in microstructure after the samples underwent freeze-thaw or wet-dry cycling tests, scanning electron microscopy (SEM) tests were performed at 1,000× magnification on both the untreated samples and those after testing, with the results depicted in Figure 9.Figure 9 Microstructural images of the sample (1000x)

(A) Microstructural images of the untreated sample.

(B–H) Microstructural images of the sample after 72 cycles under different experimental conditions: (B) under dry freeze-thaw conditions; (C) under water-saturated freeze-thaw conditions; (D) under freeze-thaw conditions in a Na₂SO₄ salt solution with pH 3, (E) pH 7, and (F) pH 12; (G) under wet-dry conditions in distilled water; (H) under wet-dry conditions in an acidic solution.

As illustrated in Figure 9A, the surface of the untreated sandstone samples appears relatively smooth and dense. Here, mineral particles show flat and irregular shapes with no visible stratification, exhibiting poor roundness, small surface pits, and fewer developed pores and cracks. Referring to Figure 9B, the surface of samples after the dry FTC is characterized by a dense population of tiny pores and a slight development of cracks; flake-off particle debris accumulates on particle surfaces or within cracks, alongside some alteration products adhering to the surface of the debris. Subsequently, Figures 9C–9F indicate that samples subjected to water-saturated and salt-resistant FTCs demonstrate a looser structure, with increased surface cracks and pores. Some fissures are observed to extend and connect, forming longer linear or irregularly net-shaped large cracks; notably, the salt-resistant freeze-thaw group exhibits severe dissolution damage, visible as surface pits and salt crystals. Figure 9G reveals that after wet-dry cycles in distilled water, the sample surfaces begin to develop small pores and micro-cracks. Furthermore, from Figure 9H, the sample displays visible signs of erosion under acidic conditions. Erosion along cracks leads to expansion and a higher degree of structural looseness. Mineral particles flake off, and the dissolution of cementing substances leads to the formation of significant erosion pits. In the wet-dry test groups, it is observed that flaked-off debris and secondary minerals chaotically fill the cracks or adhere to the surface of particles.

Mineral composition analysis

To reveal the underlying causes of damage to the samples resulting from freeze-thaw and wet-dry cycling tests, XRD tests were conducted on both the untreated samples and the post-test samples, with the results showcased in Figure 10.Figure 10 XRD identification of sample debris and changes in mineral relative content under different experimental conditions

(A and B) Under 72 freeze-thaw cycles in dry and water-saturated conditions.

(C and D) Under 72 freeze-thaw cycles in a Na₂SO₄ salt solution.

(E and F) Under 72 wet-dry cycles in acidic solution and distilled water conditions.

As depicted in Figure 10, the main components of natural sandstone include quartz, plagioclase, K-feldspar, calcite, and a minimal presence of clay minerals (kaolinite, illite, etc.). After 72 cycles of drying and water-saturated freeze-thaw (Figures 10A and 10B), the relative content of mineral components in the samples remained essentially unchanged compared to the untreated sample. The main diffraction peaks of illite and kaolinite, as well as the primary and secondary diffraction peaks of quartz, exhibited a slight increase in intensity, suggesting a minor rise in their relative contents. Meanwhile, the main diffraction peak intensities of K-feldspar, plagioclase, and calcite experienced a slight decrease, reflecting a reduction in their relative contents. Following 72 salt FTCs (Figures 10C and 10D), the mineral content of the samples exhibited certain changes compared to the untreated samples, under test conditions of pH = 3, pH = 7, and pH = 12. The intensities of the principal diffraction peaks for illite, kaolinite, and quartz were enhanced, suggesting a rise in their relative amounts. Meanwhile, the dominant diffraction peak intensities of K-feldspar, plagioclase, and calcite decreased, showing a reduction in their relative content.54 After undergoing 72 dry-wet cycles (Figures 10E and 10F), compared to the untreated sample, the relationship between diffraction peak intensity and relative content changes in the samples’ mineral components55,56 exhibits a pattern similar to that observed in the FTCs. In this context, the rate of change in the relative content of minerals was significantly greater in the acidic solution subgroup than in the distilled water subgroup.

Discussion on degradation mechanisms

Integrating the experimental results of this paper with previous scholars’ research, a discussion is conducted on the degradation mechanisms of the samples during the dry, water-saturated, salt-resistant freeze-thaw, and wet-dry cycles as follows.

Under dry freeze-thaw and water-saturated freeze-thaw cyclic conditions, the damage suffered by the sandstone samples is largely attributed to the isolated effect of frost heaving. Specifically, during the freezing period, the heterogeneous nature of rock mineral components leads to the rock mineral particles experiencing uneven shrinkage in volume.57 Simultaneously, the freezing of free water in micro-pores results in about a 9% volumetric expansion,15 which in turn generates strong frost heaving forces, culminating in elastic-plastic deformation of mineral particles and resultant structural changes.58 When these frost heaving forces exceed the strength of certain fragile, cemented parts within the rock, they instigate localized internal damage.59 With the gradual increase in temperature, the melting of ice eases the frost heave forces in the pores, enabling some of the pore deformations to achieve partial recovery, but the continuous migration of moisture further contributes to the gradual accumulation of internal damage in the rock.60 Additionally, the damage induced by frost heave manifests as irreversible deformation within the sample, exerting a long-term impact on the stability of the rock structure.

According to the previous XRD analysis results, after 72 dry freeze-thaw and water-saturated FTCs, the relative content of minerals shows slight changes compared to the untreated sample. The changes are primarily attributed to the mechanical stress exerted on the mineral crystal structure by frost heaving, affecting the microstructure of the sandstone.61 Due to the differences in volume change and thermal expansion characteristics of minerals, the degree of fragmentation and exfoliation varies among different minerals. For more brittle minerals such as feldspars (K-feldspar, plagioclase) and carbonate minerals (calcite), this stress can lead to the formation of microcracks and the decomposition of the minerals. Under the influence of water, they can easily transform into more stable clay minerals or are fractured and carried away by water flow,62 which accounts for the reduction in their content. Meanwhile, mechanically stable minerals like quartz,63 kaolinite, and illite are less prone to decomposition during FTCs, instead showing a relative content increase due to the decomposition of other minerals.64 In summary, although sandstones primarily exhibit physical weathering under both freeze-thaw conditions, there is also a slight chemical weathering present, both of which together affect the condition of the sandstones.

Under the freeze-thaw cycling conditions in Na2SO4 solution, the damage to sandstone is mainly attributed to the combined effects of frost heaving, chemical dissolution, and salt crystallization.50 The mechanism of frost heave has been previously detailed. During these cycles, a sharp decrease in temperature causes Na2SO4 in the sandstone pores to absorb water, which leads to the formation of Na2SO4·7H2O and Na2SO4·10H2O crystals.65 Additionally, when the temperature drops to −20°C, the volume expansion caused by the secondary phase change of the Na2SO4 solution induces substantial crystallization pressure,66 further accelerating the damage to the sandstone. Concurrently, the chemical composition in the sandstone is altered by the salt solution via ongoing dissolution processes. Some minerals convert to ions and oxides and vanish with the flow of the solution, intensifying the combined development of micro-fractures and pores.67 The results from XRD testing indicate that the relative content of sandstone minerals certainly changes after cycling, corroborating the previous conclusions. In a strongly acidic environment (pH = 3), sandstone minerals such as K-feldspar (KAlSi3O8) and plagioclase (NaAlSi3O8) react with H+ and H2O to form illite and kaolinite, and calcite decomposes directly when combined with H+, as shown in reactions (1)∼(5).50,68,69 In a neutral Na2SO4 solution, minerals such as K-feldspar, plagioclase, and calcite might dissolve to some extent, mainly due to water molecules directly interacting with the minerals and causing the release of some ions from the mineral lattice.70 Under strong alkaline conditions (pH = 12), the sandstone minerals K-feldspar (KAlSi3O8) and plagioclase (NaAlSi3O8) react with OH− and H2O to produce illite and kaolinite, illustrated in reactions (6)∼(8)71; calcite undergoes certain chemical dissolution under alkalinity, as depicted in reaction (9).68 In conclusion, under various pH conditions, chemical weathering consistently occurs during FTCs, and physical weathering (especially frost heaving) also plays a role, together determining the changes in the physical and chemical properties of rocks.(Equation 1) CaCO3(Calcite)+2H+→Ca2++H2O+CO2

(Equation 2) 3NaAlSi3O8(Plagioclase)+2H++K+→3Na++KAl3Si3O10(OH)2(Illite)+6SiO2

(Equation 3) 2NaAlSi3O8(Plagioclase)+2H++H2O→2Na++Al2Si2O5(OH)4(Kaolinite)+4SiO2

(Equation 4) 2KAlSi3O8(K−feldspar)+2H++H2O→2K++Al2Si2O5(OH)4(Kaolinite)+4SiO2

(Equation 5) 3KAlSi3O8(K−feldspar)+2H+→2K++KAl3Si3O10(OH)2(Illite)+6SiO2

(Equation 6) 2KAlSi3O8(K−feldspar)+6OH−+2H2O→2K++Al2Si2O5(OH)4(Kaolinite)+4H2SiO42−

(Equation 7) 2NaAlSi3O8(Plagioclase)+6OH−+2H2O→2Na++Al2Si2O5(OH)4(Kaolinite)+4H2SiO42−

(Equation 8) 3KAlSi3O8(K−feldspar)+10OH−+2H2O→2K++KAl3Si3O10(OH)2(Illite)+6H2SiO42−

(Equation 9) CaCO3(Calcite)+H2O+CO2→Ca(HCO3)2

During the wet-dry cycles, the damage to the samples stems largely from swelling-shrinkage, hydrolysis, and chemical dissolution. Sandstone is affected by swelling-shrinkage in different environments. In the water absorption phase of the wet-dry cycling test, a decrease in the content of certain minerals within the sandstone coupled with an increase in clay minerals leads to the internal pores of the sandstone becoming saturated with more free water. Additionally, the thickening of the water film adsorbed by clay minerals induces volume expansion. In the dehydration phase, these hydrophilic minerals undergo volume shrinkage or fracture due to desiccation.36,72 At the same time, gases produced by the change in the phase state of water are expelled outward through fractures.73 Collectively, these processes contribute to the accumulation of plastic deformation inside the sandstone, thereby creating a damaged zone. As this plastic zone enlarges, microcracks emerge and rapidly propagate, compromising the stability of the sandstone’s internal structure.

According to XRD analysis, the wet-dry cyclic tests conducted in a distilled water environment showed slight changes in the relative content of minerals compared to the untreated sample, mainly caused by swelling-shrinkage and hydrolysis. The action of swelling and shrinking caused some unstable mineral particles to fracture and be carried away by water flow.74 Meanwhile, the periodic absorption and evaporation of water facilitated the dissolution and recrystallization of some minerals,75 such as the silicate minerals K-feldspar (KAlSi3O8) and plagioclase (NaAlSi3O8). As water permeates the sandstone, these minerals partially transform into more stable clay minerals via hydrolysis,76 as shown in Equations 10, 11, and 12,77 leading to a decrease in their content. Illite, kaolinite, and quartz, with their greater chemical stability, resist chemical weathering more effectively under the same conditions, slightly increasing their relative content.78 Therefore, it can be seen that while physical weathering is primarily observed under wet-dry cycles, chemical weathering (mineral hydrolysis) also influences the mineral composition of sandstone to various extents.(Equation 10) 2NaAlSi3O8(Plagioclase)+11H2O→2Na++Al2Si2O5(OH)4(Kaolinite)+2OH−+4H4SiO4

(Equation 11) 2KAlSi3O8(K−feldspar)+11H2O→2K++Al2Si2O5(OH)4(Kaolinite)+2OH−+4H4SiO4

(Equation 12) CaCO3(Calcite)→Ca2++CO32−

However, when the solution has a weakly acidic pH of 5, the damage is primarily caused by swelling-shrinkage, hydrolysis, and chemical dissolution. With increasing wet-dry cycles, the binding material inside the sandstone sample progressively dissolves, leading to an increase in the looseness of particles within the initially homogeneous and compact sandstone sample. Particle movement, accompanied by the flow of water molecules, increases the roughness between particles, as well as the number, concentration, and size of pores and fissures, eventually leading to the formation of transverse microcracks in the sandstone sample.56 Furthermore, minerals such as K-feldspar (KAlSi3O8), plagioclase (NaAlSi3O8), and calcite (CaCO3) are more prone to chemical dissolution in acidic conditions than illite, kaolinite, and quartz.79 Water molecules penetrate the mineral lattice, reacting with elements like aluminum, silicon, and potassium ions within the mineral, leading to the disruption of the original mineral structure and the formation of more stable clay minerals, as shown in Equations 2, 3, 4, and 5.80 Calcite, being a carbonate, reacts with H+ ions in acidic conditions to produce soluble Ca2+ and CO2, as demonstrated in Equation 1.76 Changes in the mineral composition of sandstone post-cycling in XRD provide good evidence for the described mechanisms. It is evident that although physical weathering is also occurring, chemical weathering plays a more significant role under acidic conditions.

In summary, during the processes of FTCs and wet-dry cycles, the long-term effects of swelling-shrinkage, hydrolysis, frost heave, chemical dissolution, salt crystallization, and other actions under varied combinations of effects have led to the internal linear cracks and pores of the sample becoming extensively interconnected, enlarged, and fused. This process has resulted in a gradual decline in the sample’s hardness, mass, and P-wave velocity. The transformations in linear cracks and pores are captured in the SEM photographs of the sample post-experiment, thereby providing compelling evidence for the aforementioned observations.

Conclusions

(1) In the Helankou area, extreme fluctuations in temperature interact closely with the complex chemical composition of the aquatic environment, making freeze-thaw, and chemical coupling key factors in the weathering of sandstone-petroglyph carriers. Meanwhile, periodic precipitation and evaporation lead to changes in the dry and wet environment, further influencing the weathering process of the carrier sandstones.

(2) During drying, water-saturated, salt-resistant freeze-thaw cycling, and alternating wet-dry conditions, the physical properties of the sandstone samples—such as mass, P-wave velocity, and surface hardness—consistently show significant degradation. Under freeze-thaw conditions, the degree of change in the physical indicators of the samples increases sequentially from the dry state, to the water-saturated state, and finally to the Na2SO4-enhanced conditions under varying pH levels: 7, 12, and then 3. Under wet-dry conditions, the extent of change in the parameters in an acidic environment at pH 5 is greater than that under distilled water conditions.

(3) In the course of wet-dry and freeze-thaw cycles, the mineral composition of sandstone undergoes certain modifications: the relative contents of illite, kaolinite, and quartz increase, while the relative contents of K-feldspar, plagioclase, and calcite decrease. Simultaneously, the particle size distribution becomes more concentrated and uniform as the wet-dry and freeze-thaw experiments progress.

(4) The degradation of the samples after drying and water-saturated freeze-thaw cycles is primarily due to frost heaving. The degradation following freeze-thaw cycles in Na2SO4 solutions at different pH levels results largely from the combined effects of frost heave, chemical dissolution, and salt crystallization. Moreover, the degradation of the samples after wet-dry cycles in distilled water is mainly attributed to swelling-shrinkage and hydrolysis. When conducted in an acidic solution with a pH of 5, the degradation is predominantly due to swelling-shrinkage, hydrolysis, and chemical dissolution.

Limitations of the study

The weathering of sandstone in natural settings is the result of interactions among various factors. However, this study of the degradation conditions of sandstone-petroglyph carriers mainly considered external geological forces like moisture and temperature, while overlooking internal geological forces and additional external geological forces. For instance, certain regions of the sandstone surface feature dense vegetation and widespread moss growth, indicative of biological weathering, yet this aspect of deterioration was overlooked in this study. Moreover, considering the millennia-long history of the Helankou petroglyphs, the accelerated indoor degradation simulation experiments conducted in this study lasted only 72 cycles, which is an insufficient duration to effectively simulate the long-term effects of environmental factors such as wet-dry and freeze-thaw cycles on sandstone degradation. On the other hand, the effects of salinity are typically long-term, and the direct method of immersing in salt solutions does not fully and accurately reflect the formation process of chemical pathologies in sandstone. At the same time, the challenge of using large samples of sandstone under experimental conditions has constrained our ability to consider size effects in the weathering process. Thus, while this study presents weathering mechanisms influenced by single and multiple coupled factors, validating these theoretical bases in practical work environments requires conducting more realistic weathering experiments to provide further support in protecting petroglyphs.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER	
Meteorological data	
	
Yinchuan city temperature data	China Meteorological Data Network	http://data.cma.cn/	

Resource availability

Lead contact

Requests for further information and resources should be directed to the lead contact, Faguo He (hefg@lzu.edu.cn).

Materials availability

This study did not generate new unique materials.

Data and code availability

All data reported in this paper will be shared by the lead contact upon request.

This paper does not report original code.

Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.

Experimental model and study participant details

Not relevant to this study.

Method details

Study area

The Helankou Petroglyphs Conservation Area is located in Helan County, Yinchuan City, positioned at 106°02′ E, 38°44′ N (see the below figure), with a north-south width of 5863 m, an east-west length of 4399 m, and a total area of approximately 258 square kilometers. The petroglyphs are concentrated on the rock faces on both the north and south sides of the Helankou gorge mouth, extending westward for about 600 m, totaling over 6000 petroglyphs.Geographic location map of Helankou petroglyphs

The sandstone from the native rock bodies near the Helankou petroglyphs was dried, made into thin sections, and stained, then analyzed by the Lanzhou Supervision Center of Mineral Resources, Ministry of Land and Resources using a Zeiss polarizing microscope (Scope A1 YQ158). The sandstone features a medium to fine sandy granular structure with blocky formation, consisting mainly of clastic materials and matrices, altered by metamorphic recrystallization. Detrital materials predominantly consist of quartz (Q), plagioclase (Pl), potassium feldspar (Kf), along with quartzite and mudstone fragments. Sorting and rounding are moderate, with grain sizes primarily in the fine to medium range of 0.1–0.5 mm. Detrital particles frequently display sub-angular to sub-rounded forms, with portions appearing thoroughly rounded. The matrix mainly consists of argillaceous material and siliceous, calcareous cement, with a support type of muddy particles. After recrystallization, the muddy matrix transforms into fine sericite and chlorite microflakes. These single crystals with long axes less than 0.025 mm, are aggregately distributed around the detritus. The siliceous cements crystallize into fine quartz microcrystals, or cause the edges of quartz fragments to increase autogenously. The calcareous cementing materials primarily manifest as calcite crystals with diameters ranging from 0.02 to 0.5 mm, the larger grains also displaying a detailed poikilitic structure (see the below figure).Results of the sample observation

1-Q; 2-Pl; 3-Kf.

From 1960 to 2022 (see the below figure), the monthly average temperatures in the Helankou area showed a general trend of gradual increase from January to July and a gradual decrease from August to December, exhibiting distinct seasonal characteristics. The annual average temperature was 9.15°C. The monthly average temperature changes throughout the year were relatively stable, with adjacent months showing a stable variation of ±5.63°C. The highest monthly average temperature occurred in July, and the lowest in January. The difference between the highest and lowest monthly average temperatures ranged within 27.46 ± 4°C. The extreme maximum temperature was 39.60°C, the extreme minimum temperature was −31.40°C, and the extreme temperature range was 70°C. The figure below does not include extremes.Long-term average temperature of the Helankou area

25%–75% represents the middle 50% range of temperature distribution each month; 1.5IQR refers to 1.5 times the interquartile range of temperatures each month, which is the extreme position of the lines extending from the upper and lower boundaries of the boxplot.

Field sampling

Based on the field survey of the Helankou petroglyphs and in compliance with the conservation requirements of the Helankou scenic area, several fresh samples with similar properties were selected from the native rock bodies surrounding the sandstone that serves as the medium for the petroglyphs. (see the below figure) These were used in this degradation experiment. Sampling points 1 and 2 are located at coordinates 38°44′40″N, 106°1′18″E, at altitudes of 1480m and 1478m, respectively. Sampling points 3 and 4 are located at coordinates 38°44′43″N, 106°1′12″E, at altitudes of 1485m and 1484m, respectively. Sampling points 5 and 6 are located at coordinates 38°44′46″N, 106°1′4″E, at altitudes of 1496m and 1515m, respectively.Sampling area

Experimental methods

As mentioned in the text, the samples were tested for mass, P-wave velocity, and surface hardness after undergoing freeze-thaw and wet-dry cycles. A comprehensive analysis was also carried out, encompassing particle size distribution, Scanning Electron Microscopy (SEM), and X-Ray Diffraction (XRD). The detailed methods are as follows.

The quality assessment adopts quantitative physical measurement methods. All samples are precisely weighed after drying in their initial state to establish baseline data. After every three cycles, samples are placed in an oven and dried until the mass stabilizes, then weighed using a precision electronic scale. Before measuring, it is necessary to use a soft brush to remove dust and impurities from the surface of the samples in order to ensure the accuracy of the measurements. The rock wave speed test uses an RSM-SY5 (T) type sonic instrument. After every six cycles, samples are dried to constant weight and wave speeds are measured via through-transmission, with three measurements taken per sample and averaged. The sonic instrument uses a sandwich-type longitudinal wave planar acoustic transducer, with both receiving and transmitting frequencies of 50 kHz, and a data collection interval of 1 μs. Coupling agents such as petroleum jelly are applied to the ends to eliminate air gaps, thereby improving the precision of wave speed measurements. The hardness test employs a TH-120A portable Leeb hardness tester, utilizing a single-point impact method. After every three cycles, samples are dried to a constant weight, followed by fixed multipoint testing on both the upper and lower surfaces and data recording. After discarding overly dispersed data, the mean is taken as the Leeb hardness of the sample surfaces post-cycle.

The testing of particle size distribution was carried out via wet measurement using the Mastersizer 2000 laser particle size analyzer, manufactured by Malvern Instruments Ltd., UK. Particles were extracted from the top 0–10 mm layer of sandstone samples, both untreated and after undergoing 72 degradation test cycles, once dried to a constant weight. Following the removal of organic matter by boiling and soaking the particles in a solution of hydrogen peroxide and hydrochloric acid, 10ml of 0.05 mol/L sodium hexametaphosphate dispersant was added. The sample was then processed for 10 minutes in an ultrasonic shaker to achieve uniform dispersion. Subsequently, particle size analysis was performed as per the predefined testing protocol. Images were captured using an Apreo S model scanning electron microscope for scanning electron microscopy analysis. Following drying to constant weight, samples from the upper 0–10 mm layer of untreated and 72-cycle degraded sandstone samples were shaped into thin slices smaller than 5 mm in dimensions. Microstructural photography was performed after gold sputtering for 60 s prior to testing. The X′ Pert Pro MPD powder X-ray diffractometer from PANalytical in the Netherlands is utilized for X-ray diffraction (XRD) analysis. Untreated and degradation-tested (72 cycles) sandstone samples are dried to constant weight, and samples from the surface 0–10 mm are ground into powder form (approximately 2g each) for X-ray diffraction (XRD) analysis. The test is conducted at a voltage of 40 kV, scanning between 3° and 80° (2θ) at a scan rate of 5° per minute. Following the test, the data is analyzed qualitatively for mineral composition and relative content.

Quantification and statistical analysis

This study, based on the actual environment of petroglyphs, designs freeze-thaw and wet-dry cycling tests under various conditions for sandstone samples. Through these experiments, we have precisely quantified the degree of degradation of sandstone under the effects of freeze-thaw and wet-dry cycles. The specific quantification indicators include mass loss rate, P-wave velocity reduction rate, hardness loss rate, changes in particle size distribution, and mineral composition alterations. This study does not employ complex statistical analysis methods to process the data. Instead, it focuses on meticulously documenting the macroscopic and microscopic changes in the sandstone after specific cycles. This approach aims to comprehensively analyze the patterns of performance changes and mechanisms of chemical damage in sandstone under environmental stress, from both macroscopic and microscopic perspectives, providing a theoretical basis and practical reference for the formulation of scientific conservation measures for the Helankou petroglyphs.

Additional resources

Our study has not generated or contributed to a new website/forum or not been part of a clinical trial.

Acknowledgments

This research is supported by 10.13039/100012899 Lanzhou University and funded by the 10.13039/501100012166 National Key Research and Development Program of China (no. 2023YFF0905901 ). We would like to thank the editors and reviewers for their affirmation and help in improving the quality of this article.

Author contributions

F.H. conceptualized the research and designed the research framework; X.K. performed the data analysis; R.L. performed the experiments and contributed ideas to the data analysis; X.K. and F.H. drafted the manuscript, with discussions and contributions from L.Z. and other co-authors.

Declaration of interests

The authors declare no competing interests.
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